For example, if you're not using the AIM app, you won't see any new instant messages until you open that app. But once Push Notification Service is rolled out, AIM's can send a notification of your new messages to Apple's "cloud," and then Apple will push it down to your phone. Forstall mentioned pop-up text-based notifications (like those used currently by the Calendar app), "badge" notifications (like the red circle icon on the that shows you how many unread Mail messages you have), and sounds.

Apple intentionally made sure that iPhone and iPod touch applications can't run as background processes, so users know when they quit an app, it really quit. Running background processes also saps battery life and slows down processor performance. The PNS uses Apple's always-on IP connection to the iPhone, which it "loans" to developers to let them accomplish roughly the same objectives. The final service is expected to roll out sometime in September.